In recent years, the use of containerized shipping has become a predominant mode of transporting perishable goods by overseas shipment followed by rail or truck transport from the docks to the final destination. While the containers are on board the ship, cold air produced by one or more mechanical refrigeration systems is supplied to and circulated through the shipping containers by means of upper and lower portholes in the container. However, when the container is removed from the ship, a portable and detachable refrigeration system is needed for each container while it is being shipped by land to its final destination. Also, portable refrigeration systems are required for containers shipped by air or land where ocean transport is not involved.
While portable, mechanical refrigeration systems have been used to supply this in-transit refrigeration, such mechanical refrigeration systems have a number of serious disadvantages including, for example, high cost, mechanical complexity and consequent failures, and lack of rapid cool-down capacity. These disadvantages have been largely overcome by cryogenic refrigeration systems utilizing a tank of cryogenic refrigerant such as liquid nitrogen or liquid carbon dioxide, and one such system is disclosed in U.S. Pat. No. 3,675,439. However, previous cryogenic refrigeration systems have been difficult and time consuming to mount on the shipping container since numerous connections were required between the internal and external components of the refrigeration system. For example, the presence of the standard bulkhead immediately inside the porthole of the shipping container has required that the shipping container have a permanently mounted spray header or nozzle behind the bulkhead which requires connection to the externally mounted tank of cryogenic refrigerant. Similarly, the temperature sensor located within the shipping container has required connection to the externally mounted control system. In addition, two separate operations were required to connect the porthole closure plug to the container, and separately mount the refrigeration system to the shipping container. Difficulties have also been experienced in sealing the porthole closure plug to the front wall of the shipping container due to excessive bulging or indentation of the shipping container wall surrounding the porthole.